Frequency control system



Feb. 7, 1933. H 1,896,238

FREQUENCY CONTROL SYSTEM Filed Sept. 13. 1926 IN V EN TOR.

BY ,9 d ATTORNEY Patented Feb. 7, 1933 UNITED STATES:

saezss PATENTLOFFICE,

AUGUST HUND, OFVIBE'VIHESDA, MARYLAND, nssrenon, BY Mnsnnassreminurs, To WIRED RADIO, rim, or NEW YoaK, ii. Y.-,'.A CORBORATION'QF DELAWA RE l j FREQUENCY oonrnon SYSTEM Application filed Septeinber 13,1926. Serial No. 135,241.

My invention relates broadly topiezoelectric oscillators and more particularly to circuits for sustaining constant frequency oscillations generated by piezoelectric crystals.

One of the objects of my invention is to provide a stabilized circuit arrangement for sustaining the high frequency oscillations of piezoelectric crystals and preventing. the jumping of the frequency which may occur in such circuits due to improper cutting or preparation of the piezoelectric plate. 7 Another object of my invention isto pro vide a circuit arrangement for sustaining the oscillations of piezoelectric plates whereby the tendency of frequency change due to circuit oscillations, independent of the oscillations generated by the piezoelectric plate, is reduced to a minimum.

Still another object of my invention is to provide a circuit arrangement for sustaining the oscillations of a piezoelectric device in which the characteristics of oscillations generated by the piezoelectric device, depend wholly upon the characteristics of the piezo electric device without interference from parasitic oscillations which may normally oc our in the circuit in which the oscillationsof the piezoelectric crystal are sustained.

' Still another'object of my invention is to provide a circuit for sustaining the oscillations of piezoelectric plate's'wherein an electron tube circuit is ecoupled through a high frequency electron tubeamplification system to a work circuit with special provision for eliminating thetendency of circuit oscillations in the electron tube circuits independent of the oscillations developed bythe piezoelectric plates.

My invention will be more clearly understood from the following specification by reference to the; accompanying drawing in which:

Figure 1 illustrates a circuit arrangement embodying the principles of my invention; Fig. 2 is a modified circuit arrangement showing the connection of. a piezoelectric osjcillator with a resistance coupled amplification system embodying the principles of my invention.

Heretofore in the art the characteristics of the electron tube circuits in which the oscillations of piezoelectric elements are sustained have been such that there is serious tendency for the setting up of undesired oscillations due to the constants of the circuit and independent of the oscillations developed by the piezoelectric crystal element. I have ob served in a number of circuit systems in which the oscillations of piezoelectric plates" are sustained the sudden jumping of the frequency. This jumping may bedue to the im proper cutting of 'a' piezoelectric plate, that is the thickness oscillation does not occur exactly perpendicular to the plate .sincethe plate may not havebeen properlyprepared, as set forth. inmore detail in my U. S. Patent 1,822,928, dated September 15, 1931. Three slightly. different thickn'esslvibrations can then occur and avfrequency jump toany one .of the three is possible and may ofiset'the frequency of the control circuit to which the piezoelectric plate delivers energy by a considerable number of cycles. I have found that frequency changes are due more particularly to circuit oscillations independent of the oscillations developed by the piezoelectric plate. The frequency may be entirely dependent upon the circuit constants instead of being due to the frequency of the crystal, or

anode circuit and a grid circuitwhich' are substantially in resonance for use in short wave operations that circuit oscillations occur due to the feed-back through the elec tron tube.= A conditionsuch as thisoccurring in a piezoelectric crystal control system gives rise to two currents one of which is due to the electromechanically vibratory device or crystal andthe other due to the arrangement of the oscillator circuit. The result derived is a value of frequency which is not exseparated.

By my invention Iprovide a. circuit arrangement for a plezoelectrlc crystal controlled oscillator wherein the output or anode" circuit contains substantially pure inductance'without a shunting capacity whereby the oscillations which are obtained are due to' a strong fundamental current established by oscillations developed by the piezoelectric crystal, practically free of harmonics. The anode inductance is as large as possible and self-capacity effects avoided for eliminating; the tendency toward circuit oscillations in the electron tube system. The slight sacrifice in efiiciency by virtue of the off-resonance anode circuit is compensated for by the provision of an amplification system interposed between the output circuit of the oscillator and the work circuit in which the oscillations are usually employed. 2

Referring to the drawing in more detail, refere'nce character 1 designates an electron tube having a cathode 2, a control electrode 3 and a plate element 4. The cathode 2 is heated by battery 5 under control of rheostat 6. The piezoelectricplate 7 is supported be: tween conductive-inembers' 8 and 9 and by use of a switching device 10 the piezoelectric plate may be connected in shunt between the grid and plate electrodes or between the grid and ,filament electrodes through the chokecoil 11. A'grid leak 17 is connected between the grid electrode 3 and the cathode 2 in order to maintain the grid electrode at a suitable negative potential with respect to the cathode. The plate circuit includes an inductance 12 which is made as largeas possible for the frequency for which the piezo-el'ectrio plate 7 isground. A source of potential 14: is connected in the anode circuit in series with an indicator 15" and telephone headset 16, which may be shunted by means of a bipass condenser 27. It will be observed that the inductance 12 "does not have a shunting capacity element thereacross and the selfcapacity of this element is made as small as possible. In this manner the input and output circuits are rendered nonresonant so that the tendency for self-oscillations to be set up is extremely. small. The circuit may then be set into oscillation entirely by the energy derived from the piezoelectric crystal.

In Fig. 2 I have illustrated a high resistance leak path 17 provided between the grid and filament electrodes for improving the efiiciency of the oscillator by providing a negative grid bias to facilitatethe starting of oscillations without the grid choke. The large inductance12 has the effect of keeping the oscillations steady and rendering the input and output circuits non-resonant but arput circuit of the amplifier may be tuned but the tuned circuit is so far removed from the oscillator circuit and shielded therefrom, that circuit oscillations in the oscillator circuit are not possible; The output circuit of the am plifier constituted by electron tube 37 ,includes inductance 19, shunted variable capacity 20 and high potential source 21. The work circuit connects across inductance .22 coupled with the output inductance 19.

In Fig. 2 have shown a resistance coupled amplifier system constituted by electron tubes 23, 24 and 25 interposed between the output of the oscillator and the input circuit of the tuned amplifier system. The circuits of the resistance coupled amplifier system including tubes 23, 24 and 25 are not tuned and selfoscillation is prevented. The first tuned circuit occurs in the circuit of the amplifier con stituted by tube 37 far removed from the oscillator circuit. The energy which is .finally delivered to the work circuit through minding 22 is of constant characteristic depending entirely upon the frequency of the piezoelectric plate 7' and independent of circuit oscillations. The balancing of the resistance coupled amplifier system including the circuits of tubes 23, 24 and 25 preclude the setting up of circuit oscillations for interference with the oscillations generated by the piezoelectric crystal element. In theoscillator circuit per se the non-resonant characteristics of the input and output circuits prevent regenerative effects due to circuit characteristics although they permit the sustaining of oscillations of the frequency corresponding to the frequency of the piezoelectric crystal element.

While I have described my invention in certain preferred embodiments, I desire that it be'understood that'modifications may be made and that no limitations upon my invention are intended other than are imposed by the scope of the appended claims.

What I claim as newand desire to secure by Letters Patent ofthe United States is as follows a 1. A high frequency vacuum tube oscillator comprising an electron discharge device having cathode, grid, and anode electrodes, input and output circuits therefor, both of said.

circuits being substantially non-resonant to the frequency to be generated, a piezoelectric crystal element connected to a plurality of said electrodes, said input and output circuits and the grid to anode capacity being such that at the generated frequency, energy is adapted to be transferred from said output circuit to said input circuit through a path independent of said crystal element, and an electron tube amplification system having resistance coupled-stages and an output circuit for the last stage which is tuned to the frequency of said piezocrystal element.

2. In a piezoelectric control oscillator, the combination of an electron tube having grid,

filament and plate electrodes, input andoutput circuits for said electrodes for the generation of oscillations, a piezoelectric crystal element connected to a plurality of said elec-- trodes for sustaining oscillations in said circuits corresponding to the frequency of said crystal element, said circuits being nonresonant with respect to each other at the frequency of said crystal element, and an amplifi cation system comprising a plurality of electron tubes interconnected by circuits having substantially aperiodic frequency characteristics, the output circuit of said amplification system located in a position remote from, and being tuned to the frequency of, said crystal element.

3. In a frequency control system, the combination of an electron tube having grid, filament and plate electrodes, input and output circuits of substantially aperiodic frequency characteristics connected to said electrodes for V the generation of oscillations, a piezoelectric crystal element connected with a plurality of said electrodes for determining the frequency of said oscillations, said circuits being so arranged that, at the operating frequency, en-

- ergy is adapted to be transferred from said output circuit to said input circuit through paths independent of said crystal element, means comprising a plurality of resistance coupled electron tube stages for increasing the amplitude of said oscillations, and an output circuit connected with said means at a point electrically remote from said crystal element and tuned to the frequency of said crysml element.

In testimony whereof I aflix my signature.

AUGUST HUND. 

